'03 Impala, 3400 V6 started, ran for about 3 seconds and stopped running. After that it would crank, but won't start or run. Security light flashed on, but only stayed on for a second. I have already checked for spark (good), done a security reset (still nothing), checked fuel pump relay (good), pulled positive battery cable for 30 minutes (no change). I have also used a diagnostic code unit, but it says no codes to display. I am at a loss. NEED HELP. Any suggestions would be appreciated.

'03 Impala, 3400 V6 started, ran for about 3 seconds and stopped running. After that it would crank, but won't start or run. Security light flashed on, but only stayed on for a second. I have already checked for spark (good), done a security reset (still nothing), checked fuel pump relay (good), pulled positive battery cable for 30 minutes (no change). I have also used a diagnostic code unit, but it says no codes to display. I am at a loss. NEED HELP. Any suggestions would be appreciated.

four things make a car run...

Air
Fuel
Spark
Ignition

i see you've checked spark and fuel at the pump at least... air is going in i'm guessing... is any of the injectors bad? and are all the spark plug wires sending the spark to the spark plugs and not shorting out? yes some dumb ?'s but it's the best way to find out

I have spark at all plugs. I have, I fear, narrowed it down a bit. I used starter fluid and the result was that it would start and run as long as I spray it with fluid. I appreciate the advice, but I think that pretty much narrows it down to the fuel pump. Unless you have any other recommendations, I am always open to any suggestion that might save me from spending that kind of money.

i say pull the vacume hose off your fuel pressure regulator and see if u smell gasoline, if you do, replace it. its a 30 dollar part n easy to replace and is a common problem and gives u the problem ur having when it fails

The fuel pressure regulator can be bad even if you don't smell gasoline in the vacuum line. That could be your culprit. If you replace the fuel pump, I'd replace the fuel pressure regulator, too. They are both common problems in these cars.

You can test the fuel pump and regulator using a fuel pressure gauge. One of the auto stores may rent them. If you haven't done so lately, change the fuel filter before you test the fuel pressure.